Romaine lettuce is a different cultivar of lettuce from the standard iceberg lettuce used on most Taco Bell menu items. Romaine is generally only used on the Cantina items (Power bowl, etc.), but can be substituted/added to other items.
In 2018, the Centers for Disease Control (CDC) recalled romaine lettuce nationwide due to contamination with E. coli bacteria. This recall was later specified to only apply to lettuce grown in California. Because of this recall, romaine lettuce may not be available at many Taco Bell restaurants.